Vandal from Hell power supply and trim pots questions Topic is solved

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micmac
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Post by micmac »

Hi all,

Newbie alarm, first post :)

I searched the web for some information but didn't find any build reports for the Vandal from Hell yet.

I ordered a kit of the Vandal from Hell kit from Musikding. The build document is here:

https://pcbguitarmania.com/wp-content/u ... g-Docs.pdf

PCBGuitarMan posted a picture of the PCB here on FSB in '21:

viewtopic.php?f=13&t=28945&p=286151&hilit=hell#p286151

In the above thread people wrote that they used their circuit with 12V or 18V power supplies, because the Randall circuit used 24V. But in the build doc of the Vandal from Hell it says 9V.

Also, there's two 100k trim pots mentioned in the build doc. But it doesn't say what to do with them. In the PCB layout they're marked as POT1 (close to Q1) and POT3 (close to Q2).

My two questions:

1. Is 9V the correct power supply voltage for the Vandal from Hell or should it be used with higher voltage for more headroom?
2. Are the trim pots to be used to bias Q1 and Q2 to 4,5V (half of the power supply voltage) on the transistor drains?

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mauman
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Post by mauman »

Welcome!

My experience with J201 preamps is that you can't just increase the voltage from 9V to 18V without adjusting the drain and source resistors. It won't hurt anything, but it will sound quite different than designed.

Yes, Q1 and Q2 are biased using the pair of 100k trim pots. For a 9V supply, somewhere between 4.5V and 6V at the drain is usually recommended, and you can fine-tune the drain resistor (trim pot) by ear. There's a concept called the Fetzer valve that proposes optimum bias points for various supply voltages and FET types when doing a conversion from tube design to FET design. If you like that idea, details can be found here, there's a calculator at the bottom of the page. https://www.runoffgroove.com/fetzervalve.html

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micmac
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Post by micmac »

Thank you mauman for all this information! Much appreciated!

The linked fetzervalve paper seems a bit over my head. I shall read it a couple more times :)

Musikding sent a mail that the shipment is almost on its way. So I think I should have the kit sometime next week. I'll post back once I have the device boxed up.

Thanks again for your help!

Kind regards,
Seb

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mauman
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Post by mauman »

Looking forward to hearing about it! BTW, using the Fetzer Valve approach for your build would be setting the trim pots for Q1 and Q2 closer to 6V at the drains than to 4.5V, for a 9V supply.

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